1. Field of the Invention
The present invention relates to a writable information recording medium, and, more particularly, to an information recording medium having a recording layer and a reflective layer formed on a substrate having light transmission properties.
2. Description of the Related Art
It is known that conventional writable information recording media use a recording layer essentially consisting of an organic dye, such as a cyanine type dye or a phthalocyanine type dye.
To write information on such an information recording medium, a laser beam is focused on a minute area of the recording layer so that it is transferred to thermal energy to alter the property of the recording layer, thus accomplishing the information recording (forming pits). To smoothly change the property of the recording layer, the information recording media typically employ a so-called air sandwich structure which is accomplished by preparing two sets of substrate-and-recording-layer laminations each having a recording layer formed on a transparent substrate, and arranging those two sets of lamination structures in such a way that the recording layers face each other.
A write laser beam used for this type of information recording medium is irradiated on this medium from the transparent substrate side to form optically readable pits in the recording layer. A read laser beam for reproducing the recorded data has a lower intensity than the write laser beam. When this read laser is irradiated on the information recording medium, the contrast between a pit-formed portion and a pit-absent portion is read out as an electrical signal.
There is a different type of medium on which data has been pre-recorded, a so-called ROM (Read Only Memory) type medium. The ROM type media are widely used in the fields of audio recording and information processing. This type of medium however does not have the writable recording layer mentioned above. More specifically, pre-pits corresponding to data to be reproduced have previously been formed on a plastic substrate by injection molding, and a reflective layer of a metal such as Au, Ag, Cu and Al is formed on the pits-formed substrate, with a protective layer covering the top of the reflective layer. A typical ROM type medium is a so-called CD (Compact Disk). The specifications for recording and reading signals for this CD are standardized, and compact disk players (CD players), which conform to the standards, are widely used as CD reproducing apparatuses.
The aforementioned writable information recording medium is the same as a CD in the use of a laser beam and in its disk shape. There is therefore a strong demand for the development of writable media, which conform to the standards of the CD specifications and can be thus used directly in available CD players. To meet this demand, actual samples have been gradually proposed.
An organic dye, which is the essential component in the recording layer of such a writable information recording medium, does not have sufficient light resistance. Nitroso compounds which prevent tenebrescence are used as one means to improve the light resistance.
However, there is a strong demand for media whose recording layer contains a material that is much safer and easier to be synthesized than those of a nitroso compound and further having an excellent effect of improving light resistance.